Electronic Structure Engineering of Highly‐Scalable Earth‐Abundant Multi‐Synergized Electrocatalyst for Exceptional Overall Water Splitting in Neutral Medium

Abstract Efficient neutral water splitting may represent in future a sustainable solution to unconstrained energy requirements, but yet necessitates the development of innovative avenues for achieving the currently unmet required performances. Herein, a novel paradigm based on the combination of electronic structure engineering and surface morphology tuning of earth‐abundant 3D‐hierarchical binder‐free electrocatalysts is demonstrated, via a scalable single‐step thermal transformation of nickel substrates under sulfur environment. A temporal‐evolution of the resulting 3D‐nanostructured substrates is performed for the intentional enhancement of non‐abundant highly‐catalytic Ni3+ and pSn 2− species on the catalyst surface, concomitantly accompanied with densification of the hierarchical catalyst morphology. Remarkably, the finely engineered NiS x catalyst synthesized via thermal‐evolution for 24 h (NiS x ‐24 h) exhibits an exceptionally low cell voltage of 1.59 V (lower than Pt/C‐IrO2 catalytic couple) for neutral water splitting, which represents the lowest value ever reported. The enhanced performance of NiS x ‐24 h is a multi‐synergized consequence of the simultaneous enrichment of oxygen and hydrogen evolution reaction catalyzing species, accompanied by an optimum electrocatalytic surface area and intrinsic high conductivity. Overall, this innovative work opens a route to engineering the active material's electronic structure/morphology, demonstrating novel Ni3+/pSn 2−‐enriched NiS x catalysts which surpass state‐of‐the‐art materials for neutral water splitting.


Figure S1
Cross sectional SEM image of NiS x -24h. Figure S2: (a) IV plots and (b) bar graph representation of corresponding conductivity values for different NiS x Figure S3 Cyclic voltammograms (5 mV/s) of different NiS x in the oxygen evolution region.

Figure S4
Bar graph representation of R CT values calculated for different NiS x in the oxygen evolution region. Figure S5 ΔJ versus scan rate and (b) ECSA for NiS x Ni-based sulphides in the non-faradic region.

Figure S6
ECSA normalized OER polarization curve of different NiS x in 0.5 M PBS.

Figure S7
Chronoamperometric stability measurement of NiS x -24h at 10 mA/cm 2 in the OER region. Inset in figure shows the cyclic stability.

Figure S8
Cyclic voltammograms (5 mV/s) of different NiS x in the hydrogen evolution region.

Figure S9
Bar graph representation of R CT values calculated for different NiS x in the hydrogen evolution region.   Tabulation of theoretical and experimental reports revealing Ni 3+ as the active moiety for catalyzing OER.  Tabulation of theoretical and experimental reports revealing polysulphide (S n 2-) as the active moiety for catalyzing HER.

Figure S14
Theoretical and experimental yields of hydrogen and oxygen towards overall water splitting using NiS x -24h. Figure S15 1 st and 1000 th cyclic voltammetry plot for NiS x -24h.

Theory and Experimental
Small 2018,14,1800136 [3] 4 Perovskite Oxide This pioneer work systematically examined more than 10 transition metal oxides experimentally and theoretically. The work demonstrated that the occupancy of the 3d electron with an e g symmetry has volcano-

Theory and Experimental
Science 2011, 334, 1383. [4] [5] 6 Ni-Co Oxide Ni-Co oxide nanosheet exhibits an overpotential of ≈0.34 V for OER in alkaline media. The enhanced activity is related to Ni 3+ enriched surface which benefits the formation of main redox site (NiOOH) as revealed by in-situ X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure.

Experimental
Adv. Energy Mater. 2015, 5, 1500091. [6] 7 LiNiO 2 Electronic structure of NiO was tuned via Li doping to enhance the OER activities. A synergistic combination of synchrotron-based photoemission spectroscopy, X-ray absorption spectroscopy, and density functional theory reveals that the Ni 3+ oxidation states stabilize the adsorption of OHon the electrode, hence facilitating the fast kinetics for OER.

Theory and Experimental
Chem.

Theory and Experimental
ACS Catal. 2016, 6, 861. [12] density functional theory calculation were performed. The bridged S n 2− species were found to be the HER catalyst with ΔG H* closer to zero in comparison to apical (S 2-) species. In situ Raman spectroscopy of the [Mo 3 S 13 ] 2further demonstrate the higher catalytic reactivity of the polysulphide (S n 2-) over apical (S 2-) for proton reduction.

S-rich Cobalt Polysulfide
Excellent HER performance with an overpotential of 42 mV at 10 mA/cm 2 in alkaline medium was attained. The superior performance of the S-rich CoS x composite was attributed to the unique interconnected silk-cocoon structure and the polysulfide composition.

Grapheneencapsulated CoNi
A high-performance electrolyzer was designed, using polysulfides as mediators and graphene-encapsulated CoNi as catalysts. It produced H 2 with a low potential of 0.82 V at 100 mA/cm 2 .

Experimental and theory
The Innovation 2, 2021, 100144. [14] Figure S12: Cyclic voltammograms (5 mV/s) of different NiS x in as bifunctional catalyst for overall water splitting in 0.5 M PBS.